It is well known in communication systems to communicate data using a carrier signal modulated with the data. In a conventional modulation format, such as amplitude modulation, a single carrier represents all of the data. Sub carrier Multiplexing (SCM) is a modulation format whereby the carrier transporting the data consists of a plurality of sub-carriers. SCM is used for carrying multiple data channels, typically arranged so that each channel is modulated onto an individual subcarrier. More recently systems have been built where a single data channel is spread over several subcarriers (as is the case for Orthogonal Frequency Division Multiplexing, OFDM, explained below). Each sub carrier is conventionally generated from a common source, and is modulated independently and thus represents part of the data being represented by the whole carrier.
According to known radio sub carrier systems, each sub carrier is generated by modulation of individual carriers, which are then combined to yield a sub carrier multiplexed signal. This technique has the disadvantage that individual apparatus may be provided to generate each sub-carrier, substantially increasing the cost of the system. Furthermore, guard bands between sub-carriers reduce the spectral efficiency of the modulation format, reducing the data capacity of an optical communications system.
One type of sub carrier system is an Orthogonal Frequency Division Multiplexing (OFDM) communication system. In this case, the data is spread across all the sub-carriers. FIG. 1 shows a typical spectrum of an OFDM signal, with six sub-carriers spaced in frequency. Guard bands can be provided between sub-carriers but are not shown here, since in OFDM, adjacent sub-carriers can overlap yet do not interfere with one another as they appear orthogonal when received using a FFT correctly synchronised with the symbol provided that the subcarrier spacing is the inverse of the symbol length. The modulation format utilised to modulate each sub carrier can be chosen according to the system requirements. The symbol rate of an SCM signal is therefore defined by the number of sub-carriers, and the modulation format and rate utilised for each sub-carrier. For example if four binary modulated sub-carriers are utilised the symbol rate will be a quarter of the bit rate carried by the aggregate SCM signal. Alternatively if four quadrature modulated sub-carriers are utilised, the symbol rate will be one eighth of the bit rate carried by the aggregate SCM signal.
OFDM signals exhibit high resilience to linear distortion impairments because the information is encoded in the frequency domain. These are known in radio communication systems but not in optical communication systems until the above referenced pending US patent application ref 16231ID. Optical communication systems suffer more severely from certain impairments than radio communication systems, for which the OFDM technique was initially developed. Orthogonal Frequency Division Multiplexing (OFDM) systems encode the information as a concatenation of blocks in the frequency domain. An inverse fast Fourier transform converts the information to the time domain before its transmission along the channel. A fast Fourier transform at the receiver recovers the original transmitted sequence.
If OFDM is implemented in optical communication systems as shown in the above referenced pending U.S. and international patent applications ref 16321ID, the transmitted information can be encoded in frequency by means of a given phase/amplitude modulation format like QPSK, QAM 16, QAM 32 and similar formats. Amplitude modulation formats and phase modulation formats are also possible. The polarisation dimension can also be exploited by polarisation multiplexing. For phase/amplitude modulation formats and phase modulation formats the receiver may employ a coherent-type detection scheme whereby the in-phase and quadrature components of the signal are measured. For amplitude modulation formats coherent detection is not required. Linear impairments, like chromatic and polarisation mode dispersion, can be easily equalised once the received sampled signal is converted back to the frequency domain to extract the original data.
One advantage of OFDM is that if a guard-band with cyclic prefix is included before transmission, then the received sequence is the circular convolution of the input signal with the channel response. This permits a very easy equalisation of linear impairments, as circular convolutions become products in the Fourier domain where the information is encoded. The only limitation is that the time interval over which the channel impulse-response-time extends should be smaller than the size of the allocated guard-band.
Inter channel interference, ICI, in radio transmissions is fundamentally different to ICI in optical fibers and so radio techniques are not generally appropriate for optical systems. One such difference is that the transmission medium is dispersive in optical systems, meaning that signals of different frequencies travel at different velocities. Another fundamental difference is that the nonlinear cross coupling between intensity and phase occurs along the fiber, whereas in radio systems, there is no such distributed nonlinearity. In contrast, nonlinearities in radio systems are highly localized, being caused by a very few discrete nonlinear elements. Each such localized nonlinearity is readily compensated using a discrete compensating nonlinearity. This method is not feasible in an optical fiber system, as the fiber nonlinearity interacts with the dispersion in a complex distributed manner. Hence most cross coupling or ICI in radio systems can be modeled and pre or post compensated, whereas this is not effective for optical ICI. Also, notably the bit rates in radio systems are lower and so there is more time within the bit period, for complex processing techniques. Hence where the reach performance is limited by optical nonlinearity in the fiber, current practice is to minimise nonlinear interaction between channels by spacing the channels in frequency and using chromatic dispersion to minimise the nonlinear interaction/cross coupling (by walk-off/dephasing). Where the modulation format uses multiple-phases (such as QPSK), it is currently necessary to reduce the operating power levels to minimize the impairments due to nonlinear coupling between these nominally orthogonal phase channels.
As a low power level limits the reach of the systems or increases the bit error rate, such nonlinear interactions remain a problem.